Known in the art is a method for preparing 2,6-ditert.-butylphenol by way of alkylation of phenol with pure isobutylene in the presence of a catalyst, i.e. aluminium dissolved in phenol (cf. FRG Pat. No. 1,137,444; Stroh R, Seydel R, Hohn W, Angew. Chem. 69, 6991, 1957).
In this method ortho-tert.butylphenol and 65 to 70 mol.% of 2.6-ditert.butylphenol are obtained. This method has a disadvantage residing in a high cost of the employed pure isobutylene which is recovered from isobutylene-containing hydrocarbons by special sophisticated methods.
Known in the art is alkylation of phenol with normal butenes at a temperature of from 140.degree. to 240.degree. C. in the presence of a catalyst, i.e. aluminium dissolved in phenol. At the temperature of 150.degree. C. the yield of 2-sec.butylphenol is 27.5 mol.% (cf. FRG Pat. No. 2,333,745).
Further known is a method for preparing 2,6-ditert.butyl phenol in a mixture with 2-tert.butylphenol. Alkylation of phenol is effected with a mixture of isomeric butenes containing about 50 molar % of isobutylene in the presence of a catalyst, i.e. aluminium dissolved in phenol, and at a temperature within the range of from 70 to 250.degree. C., preferably from 90.degree. to 130.degree. C.
This method has a disadvantage residing in the formation, in the process, of a by-product, i.e. 2-sec.butylphenol in a conentration which depends on the content of isobutylene in the starting mixture of isomeric butenes.
In Table 1 data are shown which demonstrate selectivity of the alkylation process depending on the starting and final content of isobutylene in the mixture of isomeric butenes.
Table 1 ______________________________________ Isobutylene concentra- Concentration of the tion in the mixture of final products, mol.% Test Temp. isomeric butenes, mol.% 2,6-ditert. 2-sec.butyl- No. .degree. C Starting Final butylphenol phenol ______________________________________ 1 110 45.4 20.4 65.5 0.7 2 110 45.4 4.1 66.2 3.2 ______________________________________
As it is seen from the Table, the higher conversion of isobutylene, the greater is the amount of the by-product, i.e. 2-sec.butylphenol. The presence of said by-product results in that isobutylene contained in the mixture of isomeric butenes is used only partially which is economically inefficient. It is impossible to totally recover isobutylene at a high selectivity of the process. After the alkylation process normal butenes containing 20.4 and mol.% of isobutylene remain which do not find any wide application.